Preamplifier for high fidelity system utilizing a moving coil stereophonic pickup cartridge

ABSTRACT

A preamplifier for a high fidelity system, which utilizes a moving coil, stereophonic, pickup cartridge, provides dynamic damping of the moving coils of that stereophonic, pickup cartridge by interposing an essentially resistive, low impedance, electrical interface between that cartridge and input of that preamplifier. Also, that preamplifier provides an overall gain which is greater than the desired overall gain, so an essentially resistive interface can be used between the output of that preamplifier and the input of the amplifier to which that preamplifier is connected.

United States Patent inventor Neil R. McCanney [56] References Cited S es" UNITED STATES PATENTS Q 2 33 1968 3,079,463 2/1963 Feldman 179/1Patented Feb, 1971 3,213,198 10/1965 Claras 179/1- Assignee Pits MusicC0.,d.b.a. Best Sound Company OTHER REFERENCES St. Louis, Mo. Tremaine,H.M. The Audio Cyclopedia H.W. Sams & acorporation Co. N.Y. FirstEdition April 1939 Pages 104,105,341 & 343.

Primary Examiner-Bernard Konick I Assistant Examiner Raymond F.Cardillo, Jr. Attme vKingsland, Rogers, Ezell, Eilers and RobbinsABSTRACT: A preamplifier for a high fidelity system, which PICK UPCARTRIDGE utilizes a moving coil, stereophonic, pick-up cartridge, pro-7 Cl 4D Fi vides dynamic damping of the moving coils of that stereo-- mmg phonic, pick-up cartridge by interposing an essentially U.S.Cll79/100.4, resistive, low impedance, electrical interface between that179/ 1 cartridge and input of that preamplifier. Also, that preamplifi-Int. Cl Gllb 3/00, er provides an overall gain which is greater than thedesired H03f 13/00 overall gain, so an essentially resistive interfacecan be used Field ofSearch 179/1A, between the output of thatpreamplifier and the input of the 100.4, 100.4ST, 1B, 100.41ST, 180.4Ed,1.3 amplifier to which that preamplifier is connected.

W 78 J2 '6 13 6'3 22 WV J0 I 86 I [a I 47 I ma fi l 20 i /00 I 7 /02 2f76 7a /9# [50 /X? /J' J /6 /z x wf 8 'M/( 6 38 PATENTEU FEB 9 I97! I l a1 I l 20 50 I00 200 500 1000 2000 5000 /0000 20000 CPS l I I 20 5 0 I00200 500 /000 2000 5000 /0000 20000 PREAMPLIFIER FOR HIGH FIDELITY SYSTEMUTILIZING A MOVING COIL STEREOPI-IONIC PICKUP. CARTRIDGE This inventionrelates to improvements in Preamplifiers. More particularly, thisinvention relates to improvements in preamplifiers for high fidelitysystems which utilizes a moving coil, stereophonic pickup cartridges.

It is, therefore, an object of the present invention to provide animproved preamplifier for a high fidelity system which utilizes'a movingcoil, stereophonic, pickup cartridge.

The use of moving coil, stereophonic, pickup cartridges in high fidelitysystems is desirable because such cartridges exhibit a high degree ofcompliance, and thus minimize the wearing of the microgrooves of thestereophonic discs with which they are used. However, the high degree ofcompliance of such cartridges reduces the trackability of thosecartridges, because it can permit the modulated walls of the microgrooveof a stereophonic disc, which was recorded at high intensity levels, todevelop sufficient momentum in the stylus and moving coils of such acartridge to cause that stylusto overshoot its intended path of traveland thereby cause the moving coils of that cartridge to produce anelectrical output which differs somewhat from the electrical outputwhich those moving coils should produce. The deviation between theelectrical output which the moving coils of that cartridge shouldproduce and the electrical output which those coils actually produce asthe stylus overshoots is referred to as transient distortion. A step-uptransformer that is presently being marketed for use with moving coilstereophonic pickup cartridges is unable to prevent the transientdistortion which occurs when the modulated walls of the microgroove in astereophonic disc cause the stylus to overshoot. It would be desirableto provide a preamplifier, for use with a moving coil, 7

therefore, an object of the present invention .to provide apreamplifier, for use with a moving coil, stereophonic, pickupcartridge, which will enable the stylus of that cartridge to faithfullyfollow the modulations on the walls of the microgroove of a stereophonicdisc even where that disc was recorded at relatively high intensity.

The preamplifier provided by the present invention makes it possible forthe stylus of a moving coil, stereophonic, pickup cartridge tofaithfully follow the modulations on the walls of the microgroove of astereophonic disc, even where that disc was recorded at relatively highintensity levels because that preamplifier provides dynamic damping ofthe moving coils of that cartridge. That dynamic damping is essentiallyuniform throughout the range of frequencies to which the moving coils ofthe moving coil, stereophonic, pickup cartridge must respond, and hencethe preamplifier provided by the present invention can enable the stylusof that cartridge to faithfully follow the modulations on the walls ofthe microgroove of a stereophonic disc, even where that disc wasrecorded at many different frequencies and at many different intensitylevels. The preamplifier of the present invention is. able to providethis desirable dynamic damping because it interposes an essentiallyresistive electrical interface between each moving coil of the movingcoil, stereophonic, pickup cartridge and the corresponding section ofthat preamplifier, and because it makes the impedance of the electricalinterface smaller than the impedance of the moving coil of to which itis connected. As a result, each moving coil of the moving coil,stereophonic, pickup cartridge will coact with the electrical interfaceto which it is connected to form a closed loop, transducer-load systemwherein the impedance of the load is less than the impedance of thetransducer: and that closed loop, transducerload system will provide thedesired dynamic damping of that moving coil. It is, therefore, an objectof the present invention to provide a preamplifier for a moving coil,stereophonic, pickup cartridge which interposes an essentially resistiveelectrical interface between each moving coil of that moving coil,stereophonic, pickup cartridge and the corresponding section of thatpreamplifier, and which makes the impedance of that electrical interfacesmaller than the impedance of the moving coil to which it is connected.

The amount of dynamic damping provided by the preamplifier of thepresent invention is small enough to enable the moving coil,stereophonic, pickup cartridge to retain a desirably high degree ofcompliance. As a result, the stylus of that cartridge will not undulywear the walls of the microgrooves of the stereophonic disc with whichit is used. This means that the preamplifier of the present inventionenables the moving coil, stereophonic, pickup cartridge with which it isused to have a desirably high degree of compliance, and yet to have goodtrackability.

The preamplifier provided by the present invention can have the outputthereof connected to the input of many different amplifiers for highfidelity systems. This is possible because that preamplifier interposesessentially resistive interfaces between the last stages ofamplificationof the sections thereof and the corresponding inputs of the amplifierdriven by that preamplifier. Those essentially resistive interfaces willdissipate some power; but the preamplifier of the present inventionprovides a total gain which equals the desired gain plus an additionalamount of gain corresponding to the power which is dissipated in theessentially resistive interfaces. it is. therefore, an object of thepresent invention to provide a preamplifier for high fidelity systemswhich can have the output thereof connected to the inputs of variousamplifiers, because it interposes essentially resistive interfacesbetween the last stages of amplification of the sections thereof and theinputs of those amplifiers.

Moving coil, stereophonic, pickup cartridges develop extremely low valueoutput signalsone moving coil, stereophonic pickup cartridge that ispresently being marketed producing an output signal having a nominalvalue of forty millionths of a volt. The manufacturer of that movingcoil, stereophonic, pickup cartridge has recommended the connecting ofthe input terminals of a step-up transformer to the output terminals ofthat moving coil, stereophonic, pickup cartridge to develop a signalwith an appreciably higher voltage. However, such step-up transformerstend to cause the signals at the output terminals thereof to haveunacceptable amounts of hum and noise therein. In addition, where thosestep-up transformers are mounted within the moving coil. stereophonic,pickup cartridges, those transformers unduly increase the weights ofthose cartridges; and where those stepup transformers are spaced fromthose cartridges, the leads between those cartridges and those step-uptransformers tend to pick up hum and transient noises. As a result, theuse of step-up transformers with moving coil, stereophonic, pickupcartridges is not very satisfactory. The preamplifier of the presentinvention obviates all need of a step-up transformer, and thereby avoidsthe hum and noise which the use of such a transformer entails. Inaddition, that preamplifier utilizes inverse feedback to largely cancelout signals which are due to noise and transients; and the first stagesof amplification of the sections of that preamplifier utilizetransistors which have the emitters thereof directly grounded tominimize the generation of noise. As a result, that preamplifier is ableto provide output signals which are essentially hum-free and areessentially distortion-free. It is, therefore, an object of the presentinvention to provide a preamplifier for moving coil, stereophonic,pickup cartridges which obviates all need of a step-up transformer,which utilizes inverse feedback to largely cancel out signals due tonoise and transients, and which utilizes transistors in the first stagesof amplification of the sections thereof that have the emitters thereofdirectly grounded to minimize the generation of noise.

Other and further objects and advantages of the present invention shouldbecome apparent from an examination of the drawing and accompanyingdescription.

In the drawing and accompanying description, a preferred embodiment ofthe present invention is shown and described, but it is to be understoodthat the drawing and accompanying description are for the purpose ofillustration only and do not limit the invention and that the inventionwill be defined by the appended claims.

In the drawing:

FIG. 1 is a schematic diagram of one preferred embodiment ofpreamplifier that is made in accordance with the principles andteachings of the present invention;

FIG. 2 is a graph showing the frequency response of the preamplifiershown in FIG. 1;

FIG. 3 is a graph showing the frequency response of a stepup transformerwhich is used as a preamplifier for a moving coil, stereophonic, pickupcartridge, and

FIG. 4 is a graph showing the levels of the second harmonic distortiondeveloped by the preamplifier shown in FIG. 1 and by the step-uptransformer which is used as a preamplifier for the moving coil,stereophonic, pickup cartridge.

Referring to the drawing in detail, the numeral denotes one of the coilsof a moving coil, stereophonic, pickup cartridge for a highfidelitysystem, and the numeral 120 denotes the other of the movingcoils of that cartridge. The coil 20 will develop a signal correspondingto one of the stereophonic channels recorded on a stereophonic disc; andthe coil 120 will develop a signal corresponding to the other of the sttreophonic channels recorded on that disc. The terminals 22 and 24 of thecoil 20 are connectable to the input terminals 26 and 28 of one sectionof a preamplifier 27; and the terminals 122 and 124 are connectable tothe input terminals 126 and 128 of the other section 125 of thatpreamplifier.

Junctions 30 and 32 connect a low value resistor 34 across the inputterminals 26 and 28. The numeral 36 denotes an NPN transistor; and acapacitor 38 and a junction 40 connect the junction 30 to the base ofthattransistor. The junction 32 is connected to the emitter of thetransistor 36 and to ground by a junction 42. A resistor 44 has oneterminal thereof connected to the base of the transistor 36 by thejunction 40, and has the other terminal thereof connected to thecollector of that transistor by a junction 46. A junction 48, a resistor50, a junction 52, a resistor 56, junctions 58 and 60, a resistor 63, ajunction 64, and a single pole, single throw switch 66 selectivelyconnect the collector of the transistor 36 to the positive terminal of abattery 68. The negative terminal of that battery is grounded. Acapacitor 54 is connected between the junction 52 and ground; and acapacitor 62 is connected between the junction and ground.

A capacitor 70 and a junction 100 connect the junction 48 to the base ofan NPN transistor 74. A resistor 76 is connected between the junction 72and ground; and a resistor 78 is connected between the emitter of thetransistor 74 and ground. A resistor 80 has one terminal thereofconnected to the base of the transistor 74 by the junction 72, and ithas the other terminal thereof connected to the collector of thattransistor by a junction 82. The junction 82, a junction 84, and aresistor 86 coact with junctions 58 and 60, resistor 63, junction 64 andthe switch 66 to selectively connect the collector of the transistor 74to the positive terminal of the battery 68. A capacitor 88 connects thejunction 84 to the left-hand terminal of a T-pad which includesresistors 90, 94 and 96 and a junction 92. A junction 98 connects thelower terminal of that T-pad to ground and to an output terminal 102;and the righthand terminal of that T-pad is connected to output terminalof the preamplifier 27. The numerals 104 and 106 denote two of the inputterminals of an amplifier 108 to which the output terminals 100 and 102of the preamplifier 27 will be connected.

Junctions and 132 connect a low value resistor 134 across the inputterminals 126 and 128 of the section 125 of the preamplifier 27. Thenumeral 136 denotes an NPN transistor; and a capacitor 138 and ajunction 140 connect the junction 130 to the base of that transistor.The junction 132 is connected to the emitter of the transistor 136 andto ground by a junction 142. A resistor 144 has one terminal thereofconnected to the base of the transistor 136 by the junction 140, and ithas the other terminal thereof connected to the collector of thattransistor by ajunction 146. A junction 148. a resistor 150, a junction152, a resistor 156, junctions 158 and 160, a resistor 163, the junction64 and the switch 66 selectively connect the collector of the transistor136 to the positive terminal of the battery 68. A capacitor 154 isconnected between the junction 152 and ground; and a capacitor 162 isconnnected between the junction and ground.

The numeral 174 denotes an NPN transistor; and a capacitor and ajunction 172 connect the junction 148 to the base of that transistor. Aresistor 176 is connected between the junction 172 and ground; and aresistor 178 is connected between the emitter of the transistor 174 andground. A resistor has one terminal thereof connected to the base of thetransistor 174 by the junction 172, and it has the other terminalthereof connected to the collector of that transistor by a junction 182.A junction 184 and a resistor 186 coact with junctions 158 and 160,resistor 163, junction 64 and switch 66 to selectively connect thecollector of transistor 174 to the positive terminal of the battery 68.A capacitor 188 connects the junction 184 to the left-hand terminal of aT-pad which consists of resistors 190, 194 and 196 and a junction 192.The lower terminal of that T-pad is connected to ground and to an outputterminal 202 by a junction 198; and the right-hand terminal of thatT-pad is directly connected to the output terminal 200 of thepreamplifier 27. The numerals 204 and 206 denote two further terminalsof the amplifier 108 to which the output terminals 200 and 202 will beconnected.

Although the components of the two sections of the preamplifier 27 couldbe. given different values, the following values have been found to beextremely useful and desirable:

Component Value Resistors 34 and 134 15 ohms Capacitors 38 and 138 I00microfarads Transistors 36 and 136 2N i010 Resistors 44 and 144 390,000ohms Resistors 50 and 150 5,600 ohms Capacitors 54 and 154 250microfarads Resistors 56 and 156 47,000 ohms Capacitors 62 and 162 2,000microfarads Resistors 63 and 163 680 ohms Battery 68 9 volts Capacitor70 50 microfarads Transistors 74 and 174 2N1 306 Resistors 76 and 17622,000 ohms Resistors 78 and 178 100 ohms Resistors 80 and 180 120,000ohms Resistors 86 and 186 6,800 ohms Capacitors 88 and 188 50microfarads Resistors 90 and 190 18,000 ohms Resistors 94 and 194 18,000ohms Resistors 96 and 196 43,000 ohms The two sections 25 and 125 of thepreamplifier 27 shown in FIG. 1 are identical; both with regard to theconfigurations thereof and with regard to the values of the componentsthereof, and hence only one of those sections need be described indetail. Those sections of that preamplifier will be housed within thesame overall casing, but a shield 210 of ferrous metal will beinterposed between those sections to minimize cross talk between thechannels of stereophonicallyrecorded information passing through thosesections.

In the operation of the preamplifier 27 shown in FIG. 1. both of themoving coils 20 and 120 of the moving coil. stereophonic, pickupcartridge will develop signals as the walls of the microgroove in astereophonic disc move relative to the stylus of that moving coil,stereophonic, pick up cartridge. The terminals 22 and 24 will apply thesignals developed by the moving coil 20 to the input terminals 26 and 28of section 25; and the terminals 122 and 124 will apply the signalsdeveloped by the moving coil 120 to the input terminals 126 and 128 ofthe section 125. The following description of the operation of section25 of the preamplifier 27 can also be considered to be a description ofthe operation of section 125 of that preamplifier.

The signals which the moving coil 20 develops, as a microgroove in astereophonic disc moves relative to the stylus of the moving coil,stereophonic, pickup cartridge, will be applied to the resistor 34 byterminals 22 and 24, input terminals 26 and 285 and junctions 30 and 32.The resistor 34 constitutes an electrical interface between the movingcoil 20 and the first stage of the section 25 of the preamplifier 27;and that resistor and that moving coil coact to constitute a closedloop, transducer-load system. in the preferred embodiment ofpreamplifier shown in FIG. 1, the impedance of the resistor 34 is l/bhms and thus is less than the 2 ohms output impedance of the movingcoil 20; and hence that moving coil and that resistor constitute aclosed loop, transducer-load system wherein the impedance of the load isless than the impedance of the transducer. As a result, the resistor 34will coact with the moving coil 20 to provide dynamic damping of thestylusinduced movement of that moving coil. That dynamic damping isimportant; because it will permit the stylus of the moving coil,stereophonic, pickup cartridge to remain in engagement with both wallsof the microgroove in a stereophonic disc almost continuously; and hencethat resistor enables the moving coil, stereophonic, pickup cartridge tohave a better transient response than it could have if it were connectedto a stepup transformer. The impedance of the resistor 34 is, however,high enough to permit the moving coil, stereophonic, pickup cartridge tohave sufficient compliance to keep the stylus of that moving coil,stereophonic, pickup cartridge from causing undue wearing of themicrogroove in a stereophonic disc.

Whenever the switch 66 is closed, current will flow from the positiveterminal of the battery 68 via that switch, junction 64, resistor 63,junctions 60 and 58, resistor 56, junction 52, resistor 50, junctions 48and 46, resistor 44, junction 40, the base-emitter circuit of transistor36, junction 42, and ground to the negative terminal of that battery;and that flow of current will render that transistor conductive. Also,current will flow from the positive terminal of battery 68 via switch66, junction 64, resistor 63, junctions 60 and.58, resistor 86,junctions 84 and 82, resistor 80, junction 72, the base-emitter circuitof transistor 74, resistor 78, and ground to the negative terminal ofthat battery; and that flow of current will render the transistorconductor 74 conductive. The 390,000 ohm value of the resistor 44enables the transistor 36 to have a Q point which causes the first stageof amplification of the section 25 to saturate about 60 decibels above anormal input of 40 microvolts. That 0 point is not symmetrically locatedbetween the saturation and clipping levels of that first stage ofamplification; but it makes the operation of that first stage ofamplification about 6 decibels quieter than if a symmetrical 0 point wasused.

The capacitor 38 and the junction 40 will couple any voltage, which themoving coil 20 develops across the resistor 34, to the base of thetransistor 36. If the coil 20 develops a signal across the resistor 34which provides a positive-going voltage at the junction 30, thecapacitor 38 and the junction 40 will couple that voltage to the base ofthe transistor 36 and thereby render that transistor more conductive.However, if the coil 20 develops a signal across the resistor 34 whichprovides a negative-going voltage at the junction 30, the capacitor 38and the junction 40 will couple that voltage to the base of thetransistor 36 and thereby render that transistor less conductive. Theinput impedance of a 2Nl0l0 transistor connected in the common-emitterconfiguration is about 900 ohms and the capacity of the capacitor 38 isone hundred microfarads; and the resulting time constant of thatcapacitor and the input impedance of that transistor is long enough toprovide good coupling and a good frequency response at frequencies aslow as cycles per second.

The emitter of the transistor 36 is connected directly to ground by thejunction 42, and that is desirable. By omitting an emitter resistor forthe transistor 36, the present invention obviates the electrical noisewhich would arise from the thermal agitation that occurs in carbonresistors. Consequently, the first stage of amplification of the section25 provides desirably quiet operation.

if the coil develops a signal across the resistor 34 which provides apositive-going voltage at the base of transistor 36, that transistorwill become more conductive; and the resulting decrease in voltage dropacross the collector-emitter circuit of that transistor will cause thevoltage at the junction 48 to become less positive. The capacitor 70 andthe junction 72 will couple the resulting negative-going voltage to thebase of the transistor 74, with a consequent decrease in theconductivity of that transistor. The resulting increase in the voltagedrop across the collector-emitter circuit of that transistor willdevelop a positive-going signal at the collector of that transistor, andthus at the junction 84. Because of the gain provided by the transistors36 and 74 of the two stages of amplification of the section 25 of thepreamplifier 27, the positive-going signal at the junction 84 will bevery much greater than the positive-going signal at the junction 30. Thetotal gain of the two stages of amplification of the section 25 isgreater than the gain which is needed to enable the output signal ofthat section to drive the amplifier 108; but that total gain is madehigh so the T-pad, which consists of resistors 90, 94, and 96 and thejunction 92, can be used as an essentially resistive interface betweenthe second stage of amplification of the section 25 and the amplifier108.

The use of the resistor 44 as a biasing resistor is desirable; becausethat resistor provides a small amount of inverse feedback from thecollector to the base-emitter circuit of the transistor 36; and thatinverse feedback will tend to attenuate any noise or transients reachingthat transistor. Specifically, if any noise or transients develop asignal at the base of the transistor 36 which is positive-going innature, the resulting increase in conductivity of that transistor willdevelop a negative-going signal at the collector of that transistor; andthe resistor 44 will feed that negative-going signal back to the base ofthat transistor and thereby tend to cancel the positive-going signaldeveloped by the noise and transients. The use of the resistor as abiasing resistor is desirable for the same reasonthat resistor providinga small amount of inverse feedback from the collectorto the base-emittercircuit of the transistor The resistor 50 has a relatively low value,and thus provides a relatively low output impedance for the first stageof amplification of the section 25 of the preamplifier 27. This is afurther reason why that first stage of amplification provides desirablyquiet operation.

The value of the resistor 76 is selected to provide a good match for theoutput impedance of the first stage of the section 25. The resistor 78acts as an emitter resistor for the transistor 74; and that resistorwill improve the thermal stability of the second stage of amplificationof the section 25. That resistor will, because of the thermal agitationtherein, introduce a certain amount of noise into the operation of thesecond stage of amplification of section 25; but the signal which isapplied to the base of the transistor 74 has such a good signal-to-noiseratio that noise is not a serious problem in that second stage ofamplification. The resistor 78 also provides additional inverse feedbackbecause it does not have a capacitor connected in parallel with it, andthus helps improve the frequency response of that second stage ofamplification. Specifically, if any noise or transients develop apositive-going signal at the base of the transistor 74, the resultingincrease in conductivity of that transistor will make the emitter ofthat transistor more positive relative to the ground, and thus willprovide a negative-going signal at the base of that transistor whichwill tend to cancel the positive-going signal developed by the noise ortransients.

The resistor 56 and the capacitor 54 constitute a decoupling networkwhich will keep noise, transients, and cross talk from reaching thetransistor 36. That noise and those transients could come from thesecond stage of amplification of the section 25 or from the section 125,and that cross talk would come from the section 125. The resistor 56also helps set the normal voltage at the collector of the transistor 36at about Wyolts; and such a low collector voltage fosters quietoperation of the first stage of amplification of the section 25 of thepreamplifier 27.

The capacitor 62 has a very low impedance to noise, transients and crosstalk, and that capacitor is connected in parallel with theseries-connected resistor 86, the collectoremitter circuit of transistor74, and resistor 78. As a result, that capacitor largely bypasses toground any noise, transients and cross talk, thereby providing goodvoltage regulation and good decoupling between the section 25 and thesections 125. The resistor 63 is a current-limiting resistor whichprotects the battery 68 against undue current drain during any periods,such as turn-on, when the capacitor 62 is not charged and thusessentially acts as a impedance.

The T-pad, which consists of resistors 90, 94, and 96 and the junction92, is an essentially resistive interface between the second stage ofamplification of the section 25 and the amplifier 108; and that T-padprovides an eight decibel attenuation of all signals applied to it. Thatattenuation improves the si ,nal-to-noise ratio of the output signal;and the use of such an interface makes it possible for the section 25 tobe coupled to almost any commercially acceptable, high fidelityamplifier.

As pointed out hereinbefore, the operation of the section 125 of thepreamplifier 27 will be essentially identical to the operation of thesection 25 of that preamplifier. This means that the resistor 134 willact as an essentially resistive interface between the moving coil 120 ofthe moving coil, stereophonic, pickup cartridge and the first stage ofamplification of the section 12.5, and will coact with that moving coilto constitute a closed loop, transducer-load system wherein theimpedance of the load is less than the impedance of the transducer. As aresult, the resistor 134 will coact with the moving coil 120 to providedynamic damping of the stylus-induced movement of that moving coil. Thatdynamic damping is important; because it will enable the stylus, towhich the moving coils 20 and 120 are attached, to faithfully follow themodulations on the walls of the microgroove in the stereophonic disc,even when the coil 120 senses stereophonicalIy-recorded signals whichwere recorded at high intensity levels.

The 390,000 ohm value of the resistor 144 enables the transistor 136 tohave a 0 point which causes the first stage of amplification of thesection 125 to saturate about 60 decibels above a normal input of 40microvolts. That Q point is not symmetrically located between thesaturation and clipping levels of that first stage of amplification; butmakes the operation of that first stage of amplification about 6decibels quieter than if a symmetrical Q point was used.

Because of the gain provided by the transistors 136 and 174 of the twostages of amplification of the section 125 of the preamplifier 27, thetotal gain of those two stages of amplification is greater than the gainwhich is needed to enable the output of that section to drive theamplifier 108; but that total gain is made high so the T-pad, whichconsists of resistors 190, 194 and 196 and the junction 192, can be usedas an essentially resistive interface between the second stage ofamplification of the section 125 and the amplifier 108. That T-padprovides and eight decibel attenuation of all signals applied to it, andthat attenuation improves the signalto-noise ratio of the output signalof the section 125. The use of such an interface makes it possible forthe section 125 to be coupled to almost any commercially acceptable,high fidelity amplifier.

By using the preamplifier 27 shown in FIG. 1, it is possible to greatlyincrease the trackability of a moving coil, stereophonic, pickupcartridge while keeping the compliance of that cartridge at a desirablehigh level. Where a step-up transformer has been used to amplify thesignals from that moving coil stereophonic, pickup cartridge, and wherea tracking force of 2 grans has been used. the stylus of that pickupcartridge has not faithfully followed the modulations of the walls ofthe microgrooves of stereophonic discs whenever that stylus sensedstereophonic modulations which were recorded at intensity levels in therange of l l centimeters per second. In contrast, where the preamplifierprovided by the present invention has been used as the preamplifier forthat same stereophonic pickup cartridge, the stylus of thatstereophonic, pickup cartridge has been able to faithfully follow themodulations on the walls of the microgrooves of the stereophonic discseven when it sensed stereophonic modulations which were recorded atintensity levels in the range of twenty centimeters per second.

In addition, the frequency response of the preamplifier provided by thepresent invention is vastly superior to the frequency response of thestep-up transformer which has been used to amplify the signals frommoving coil, stereophonic, pickup cartridges. For example, FIG. 2 is agraph which was obtained by applying signals having the same amplitudebut having individually-different frequencies, in the range of 20 cyclesper second, to the input of the preamplifier shown in FIG. 1; and thatgraph shows that the output of that preamplifier was the same at all ofthose frequencies. In contrast, FIG.

3, which is a graph that was obtained by applying essentially 1 similarsignals to a step-up transformer which has been used as a preamplifierfor moving coil, stereophonic, pickup cartridges, shows that the outputof that stepup transformer varied widely at some of those variousfrequencies.

It should also be noted that the second harmonic distortion of thepreamplifier shown in FIG. I is very much smaller than the secondharmonic distortion of the step-up transformer which has been used toamplify signals from moving coil, stereophonic, pickup cartridges. Forexample, the upper trace in FIG. 4 represents the second harmonicdistortion of the step-up transformer which has been used to amplifysignals from moving coil, stereophonic, pickup cartridge obtained from astereophonic disc which had twenty-five hundred cycle per second tonebursts recorded on it at an intensity level of i0 centimeters persecond; and the lower trace in FIG. 4 represents the second harmonicdistortion of the preamplifier of FIG. 1 when that preamplifier was usedto preamplify essentially similar signals. The second harmonicdistortion of the step-up transfonner averaged as much as 4 16/ l00thspercent of the total output signal, whereas the second harmonicdistortion of the preamplifier of FIG. 1 was very much lessbeing only 25/ IOths percent of the total output signal. The very appreciably lowersecond harmonic content in the output signals from the preamplifier ofFIG. 1 enables that preamplifier to provide an extremely high fidelitypreamplification of the signals which it receives from a moving coil,stereophonic, pickup cartridge.

The signal-to-noise ratio of signals at the output of the preamplifierof FIG. 1 is much better than the signal-to-noise ratio of signals atthe output of a step-up transformer which is used to amplify signalsfrom moving coil, stereophonic, pickup cartridges. Specifically, testshave shown that the signal-tonoise ratio of signals at the output of thepreamplifier of FIG. 1 are 2 /ztimes better than the signal-to-noiseratios of signals at the output of such a step-up transformer. Thatbetter signalsto-noise ratio materially improves the fidelity of thesignals which can be developed by any amplifier which is driven by thepreamplifier of FIGJl.

The damping factor for the preamplifier of FIG. 1 is much higher thanthe damping factor for a step-up transformer which is used to amplifysignals from moving coil stereophonic pickup cartridges. Specifically,the ths, damping factor for the preamplifier of FIG. 1 is about I 33/l00whereas the damping factor for such a step-up transformer is onlyabout 3/ l 00ths at best, and can be as low as l5/l00ths. In addition,because the damping factor for the preamplifier of FIG. I is insensitiveto changes in frequency, that damping factor is much more desirable thanthe damping factor for such a step-up transformer, which is sensitive tochanges in frequency.

stereophonic, pick up cartridge with which it is used to have a.

high degree of compliance and while having good trackability; and itprovides an extremely good frequency response provides desirably lowsecond harmonic distortion, and provides a desirably highsignal-to-noise ratio. Consequently, the use of the preamplifierprovided by the present invention makes it possible to attain a fidelityand a listening quality superior to that previously attained with anystep-up transformer for a moving coil, stereophonic, pickup cartridge.

Whereas the drawing and accompanying description have shown anddescribed a preferred embodiment of the present invention, it should beapparent to those skilled in the art that various changes may be made inthe form of the invention without affecting the scope thereof.

lclaim:

1. A preamplifier for a moving coil stereophonic pick up cartridgehaving two moving coils, one'for each of two channels, which comprises:

a first channel preamplifier section connected to one of the movingcoils of said moving coil stereophonic, pickup cartridge to receive andamplify signals generated by said one moving coil;

a second channel preamplifier section connected to the second movingcoil of said moving coil stereophonic, pickup cartridge to receive andto amplify signals generated by said second moving coil;

a resistive electrical interface connected across the input of saidfirst channel preamplifier section and connected across the output ofthe first said moving coil of said moving coil, stereophonic, pickupcartridge to coact with said moving coil to form a closed loop,-transducer-load system;

said resistive electrical interface having an impedance smaller than theimpedance of the first said moving coil of said moving coilstereophonic, pick up cartridge, whereby said electrical interface andsaid moving coil form a closed loop transducer-load system wherein theimpedance of the load is smaller than the'impedance of said transducer;

a second resistive electrical interface connected across the input ofsaid second channel preamplifier section and connected across the outputof said second moving coil of said moving coil, stereophonic, pick upcartridge to coact with said moving coil to form a closed loop,transducerload system;

said second resistive electric l interface having an impedance smallerthan the impedance of said second coil, of said moving coil,stereophonic, pick up cartridge whereby said second electrical interfaceand said second moving coil form a closed loop, transducer-load systemwherein the impedance of the load is smaller than the impedance of saidtransducer; and

whereby said electrical interfaces provide dynamic samping for saidmoving coils and the stylus connected to said moving coils.

section includes a transistor which has the emitter thereof directlyconnected to ground to obviate the thermal agitation developed withinemitter resistors, and wherein the first stage of amplification of saidsecond channel preamplifier section includes a transistor which as theemitter thereof directly connected to ground to obviate the thermalagitation develope within emitter resistors. 1

3. A preamplifier for a moving coil, stereophonic, pick up cartridge asclaimed in claim 1 wherein said first channel preamplifier sectionincludes a transistor and a coupling capacitor which directly couplessignals, developed across the first said electrical interface by saidpnemoving coil. to the base of said transistor, and wherein said secondchannel preamplifier section includes a transistor and a couplingcapacitor which directly couples signals, developed across said secondelectrical interface by said second moving coil, to the base of thesecond said transistor.

4. A preamplifier for a moving coil, stereophonic, pickup cartridge asclaimed in claim 1 wherein said first channel preamplifier sectionincludes a transistor with a biasing resistor connected between thecollector and emitter thereof to provide inverse feedback and whereinsaid second channel preamplifier section includes a transistor with abiasing resistor connected between the collector and emitter thereof toprovide inverse feedback.

5. A preamplifier for a moving coil, stereophonic, pickup cartridge asclaimed in claim. 1 wherein said first channel preamplifier sectionincludes a transistor with a biasing resistor that is dimensioned toprovide an unsymmetrical, low noise Q point for said transistor, andwherein said second channel preamplifier section includes a transistorwith a biasing resistor that is dimensioned to provide an unsymmetrical,low noise Q point for said transistor.

6. A preamplifier for a moving coil, stereophonic, pickup cartridge asclaimed in claim 1 wherein said first channel preamplifier sectionincludes a plurality of stages of amplification, wherein said secondchannel preamplifier section includes a plurality of stages ofamplification, wherein all of said plurality of stages of amplificationof the first said section are 2. A preamplifier for a moving coilstereophonic, pick up w decoupled from said second section and whereinall of said plurality of stages of amplification of said second sectionare decoupled from the first said section.

7. A preamplifier for a moving coil, stereophonic, pickup cartridge asclaimed in claim 1 wherein the total gain of said first channelpreamplifier section is greater than the gain required to develop ausable output signal for said first section, wherein the total gain ofsaid second channel preamplifier section is greater than the gainrequired to develop a usable output signal for said second section,wherein a third resistive electrical interface is connected across theoutput of said first section and is connected across an input of anamplifier to be driven by said preamplifier, wherein a fourth resistiveelectrical interface is connected across the output of said secondsection and is connected across an input of an amplifier, wherein saidtotal gain of said first section .is great enough to provide a usableoutput signal for said first section despite the signal attenuationcaused by said third electrical interface, and wherein said total gainof said second section is great enough to provide a usable output signalfor said second section despite the signal attenuation caused by saidfourth electrical interface.

1. A preamplifier for a moving coil stereophonic pick up cartridgehaving two moving coils, one for each of two channels, which comprises:a first channel preamplifier section connected to one of the movingcoils of said moving coil stereophonic, pickup cartridge to receive andamplify signals generated by said one moving coil; a second channelpreamplifier section connected to the second moving coil of said movingcoil stereophonic, pickup cartridge to receive and to amplify signalsgenerated by said second moving coil; a resistive electrical interfaceconnected across the input of said first channel preamplifier sectionand connected across the output of the first said moving coil of saidmoving coil, stereophonic, pickup cartridge to coact with said movingcoil to form a closed loop, transducer-load system; said resistiveelectrical interface having an impedance smaller than the impedance ofthe first said moving coil of said moving coil stereophonic, pick upcartridge, whereby said electrical interface and said moving coil form aclosed loop transducerload system wherein the impedance of the load issmaller than the impedance of said transducer; a second resistiveelectrical interface connected across the input of said second channelpreamplifier section and connected across the output of said secondmoving coil of said moving coil, stereophonic, pick up cartridge tocoact with said moving coil to form a closed loop, transducer-loadsystem; said second resistive electrical interface having an impedancesmaller than the impedance of said second coil, of said moving coil,stereophonic, pick up cartridge whereby said second electrical interfaceand said second moving coil form a closed loop, transducer-load systemwherein the impedance of the load is smaller than the impedance of saidtransducer; and whereby said electrical interfaces provide dynamicsamping for said moving coils and the stylus connected to said movingcoils.
 2. A preamplifier for a moving coil stereophonic, pick upcartridge as claimed in claim 1 wherein said first channel preamplifiersection includes a plurality of stages of amplification, wherein saidsecond channel preamplifier section includes a plurality of stages ofamplification, wherein the first stage of amplification of the firstsaid channel preamplifier section includes a transistor which has theemitter thereof directly connected to ground to obviate the thermalagitation developed within emitter resistors, and wherein the firststage of amplification of said second channel preamplifier sectionincludes a transistor which as the emitter thereof directly connected toground to obviate the thermal agitation developed within emitterresistors.
 3. A preamplifier for a moving coil, stereophonic, pick upcartridge as claimed in claim 1 wherein said first channel preamplifiersection includes a transistor and a coupling capacitor which directlycouples signals, developed across the first said electrical interface bysaid one moving coil, to the base of said transistor, and wherein saidsecond channel preamplifier section includes a transistor and a couplingcapacitor which directly couples signals, developed across said secondelectrical interface by said second moving coil, to the base of thesecond said transistor.
 4. A preamplifier for a moving coil,stereophonic, pickup cartridge as claimed in claim 1 wherein said firstchannel preamplifier section includes a transistor with a biasingresistor connected between the collector and emitter thereof to provideinverse feedback and wherein said second channel preamplifier sectionincludes a transistor with a biasing resistor connected between thecollector and emitter thereof to provide inverse feedback.
 5. Apreamplifier for a moving coil, stereophonic, pickup cartridge asclaimed in claim 1 wherein said first channel preamplifier sectionincludes a transistor with a biasing resistor that is dimensioned toprovide an unsymmetrical, low noise Q point for said transistor, andwherein said second channel preamplifier section includes a transistorwith a biasing resistor that is dimensioned to provide an unsymmetrical,low noise Q point for said transistor.
 6. A preamplifier for a movingcoil, stereophonic, pickup cartridge as claimed in claim 1 wherein saidfirst channel preamplifier section includes a plurality of stages ofamplification, wherein said second channel preamplifier section includesa plurality of stages of amplification, wherein all of said plurality ofstages of amplification of the first said section are decoupled fromsaid second section and wherein all of said plurality of stages ofamplification of said second section are decoupled from the first saidsection.
 7. A preamplifier for a moving coil, stereophonic, pickupcartridge as claimed in claim 1 wherein the total gain of said firstchannel preamplifier section is greater than the gain required todevelop a usable output signal for said first section, wherein the totalgain of said second channel preamplifier section is greater than thegain required to develop a usable output signal for said second section,wherein a third resistive electrical interface is connected across theoutput of said first section and is connected across an input of anamplifier to be driven by said preamplifier, wherein a fourth resistiveelectrical interface is connected across the output of said secondsection and is connected across an input of an amplifier, wherein saidtotal gain of said first section is great enough to provide a usableoutput signal for said first section despite the signal attenuationcaused by said third electrical interface, and wherein said total gainof said second section is great enough to provide a usable output signalfor said second section despite the signal attenuation caused by saidfourth electrical interface.